4L-TRAC, done right
05-13-2014, 08:00 AM
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4L-TRAC, done right
For Tacoma 2009+, possibly others.
There is a common modification used to enable the use of TRAC while having the transfer case set to low range, usually called the "yellow wire mod".
I.e., https://www.yotatech.com/forums/f166...tacoma-264329/
This modification is semi-functional, but *incorrect*.
The author of that article claims;
"*NOTE*: After doing this mod, the 4LO indicator light will FLASH when 4 LO is engaged. THIS IS NORMAL AND EXPECTED. The 4LO beeping will still occur if 4LO does not engage properly."
*** THIS IS NOT RIGHT!!!!
The flashing light is the indicator of a problem. The 4WD ECU has shifted the transfer case into low range, but has not received the signal that it actually happened!
The yellow wire mod involves clipping off pin 21-PA, which is the input signal from the low-range position sensing switch on the transfer case.
What we would really want to do, is allow that signal to act on the 4WD ECU, but keep on telling the SKID CONTROL ECU that you are actually in HIGH RANGE.
The output signal to the skid control ECU is 9-PC.
Note: On "Instrument Panel Junction Box No.2", behind the glove box, the six plugs are labelled sequentially as PA on the top (as mounted), through PF on the bottom.
Pins are numbered. As mounted, there are two vertical rows of pins, start counting pin numbers from 1 at the BOTTOM RIGHT pin, follow the row to the top, then continue from the BOTTOM LEFT and up.
9-PC is a yellow wire with a black trace.
If you clip that wire, you are, indeed, blocking the low range signal from reaching the skid control ECU, however, this will cause the skid control warning light to go on, and all electronic brake control functions to fail. The problem is that the 4WD ECU outputs a low current +12v on the low range INPUT pin. This is basically a signal to the skid control ECU that it is actually in HIGH range. When the sensor on the transfer case is activated, it provides a zero-resistance to 0v chassis, so it pulls the +12v down to 0v. The skid control ECU expects the +12.
Solution: tie that wire via 100 ohm resistor to +12. You can probably omit the resistor, but it is a cleaner solution to limit the current going in to a low current sensor.
To make it switchable, use an SPDT switch with the wire clipped off of 9-PC connected to the switch's common pin, +12v/resistor to either of the other two pins, and 9-PC to the last. Flip the switch one way and it is "normal", flip it the other way, and it is TRAC-LO. No warnings, no errors.
Note: for those with automatic transmissions, the low range signal is also sent to the ECM for altering transmission shift patterns. Doing the old yellow-wire mod will NOT inform the ECM that the transmission is in low, which means that it will be using a high-range shift pattern. This 4L-TRAC mod will MAINTAIN the low signal to the ECM, which will allow it to shift correctly for low range.
More modifications to come: 2WD-LOW
As with the 4L-TRAC, others have done this before, but incorrectly.
The correct way to do 2WD-LOW *depends* on 4L-TRAC, since both the low and 4wd signals are sent to the skid control ECU. Sending the low-range signal to the skid control ECU withOUT also sending the 4WD signal, introduces an input inconsistency. The "old way" is also timing sensitive and throws errors.
I have the modification designed, but have not yet implemented it.
There is a common modification used to enable the use of TRAC while having the transfer case set to low range, usually called the "yellow wire mod".
I.e., https://www.yotatech.com/forums/f166...tacoma-264329/
This modification is semi-functional, but *incorrect*.
The author of that article claims;
"*NOTE*: After doing this mod, the 4LO indicator light will FLASH when 4 LO is engaged. THIS IS NORMAL AND EXPECTED. The 4LO beeping will still occur if 4LO does not engage properly."
*** THIS IS NOT RIGHT!!!!
The flashing light is the indicator of a problem. The 4WD ECU has shifted the transfer case into low range, but has not received the signal that it actually happened!
The yellow wire mod involves clipping off pin 21-PA, which is the input signal from the low-range position sensing switch on the transfer case.
What we would really want to do, is allow that signal to act on the 4WD ECU, but keep on telling the SKID CONTROL ECU that you are actually in HIGH RANGE.
The output signal to the skid control ECU is 9-PC.
Note: On "Instrument Panel Junction Box No.2", behind the glove box, the six plugs are labelled sequentially as PA on the top (as mounted), through PF on the bottom.
Pins are numbered. As mounted, there are two vertical rows of pins, start counting pin numbers from 1 at the BOTTOM RIGHT pin, follow the row to the top, then continue from the BOTTOM LEFT and up.
9-PC is a yellow wire with a black trace.
If you clip that wire, you are, indeed, blocking the low range signal from reaching the skid control ECU, however, this will cause the skid control warning light to go on, and all electronic brake control functions to fail. The problem is that the 4WD ECU outputs a low current +12v on the low range INPUT pin. This is basically a signal to the skid control ECU that it is actually in HIGH range. When the sensor on the transfer case is activated, it provides a zero-resistance to 0v chassis, so it pulls the +12v down to 0v. The skid control ECU expects the +12.
Solution: tie that wire via 100 ohm resistor to +12. You can probably omit the resistor, but it is a cleaner solution to limit the current going in to a low current sensor.
To make it switchable, use an SPDT switch with the wire clipped off of 9-PC connected to the switch's common pin, +12v/resistor to either of the other two pins, and 9-PC to the last. Flip the switch one way and it is "normal", flip it the other way, and it is TRAC-LO. No warnings, no errors.
Note: for those with automatic transmissions, the low range signal is also sent to the ECM for altering transmission shift patterns. Doing the old yellow-wire mod will NOT inform the ECM that the transmission is in low, which means that it will be using a high-range shift pattern. This 4L-TRAC mod will MAINTAIN the low signal to the ECM, which will allow it to shift correctly for low range.
More modifications to come: 2WD-LOW
As with the 4L-TRAC, others have done this before, but incorrectly.
The correct way to do 2WD-LOW *depends* on 4L-TRAC, since both the low and 4wd signals are sent to the skid control ECU. Sending the low-range signal to the skid control ECU withOUT also sending the 4WD signal, introduces an input inconsistency. The "old way" is also timing sensitive and throws errors.
I have the modification designed, but have not yet implemented it.
Last edited by doitright; 05-13-2014 at 11:53 AM.
08-27-2014, 06:19 PM
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Just a little update on this:
I finally found the time to actually install and test this. Works flawlessly. Did not have to go back and re-do anything, just connected it up exactly as per these instructions.
No warnings go off. When the override circuit is turned off, turn the knob to 4wd and it goes, after the typical delay waiting for the ADD to click in. Override turned on and then switch to 4wd, it almost instantly goes to solid-4wd light -- no ADD delay. Still in 2WD. All the way to low, xfer case is happily in 4L, ADD still disconnected. 2-Low.
I finally found the time to actually install and test this. Works flawlessly. Did not have to go back and re-do anything, just connected it up exactly as per these instructions.
No warnings go off. When the override circuit is turned off, turn the knob to 4wd and it goes, after the typical delay waiting for the ADD to click in. Override turned on and then switch to 4wd, it almost instantly goes to solid-4wd light -- no ADD delay. Still in 2WD. All the way to low, xfer case is happily in 4L, ADD still disconnected. 2-Low.
10-31-2016, 12:42 PM
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Follow up question
I realize this is an old thread, so I apologize for trying to resurrect it, but...I would really like to do this modification the way you are describing. You are the only person I've seen who has figured this secondary part out.
Could you post some pics, or even better yet, a video walking me through what I need to do?
If that is not convenient, could you give some more detailed instruction? The part I got lost at was when you said:
I'm confused as to which wire is that wire, and which wire is +12? I apologize, I'm not that knowledgeable about wiring and electricity.
Also, since you have been running this setup for 2+ years, have you had any problems or unintended consequences from it?
I would greatly appreciate your help!!
Thanks,
Brian
Could you post some pics, or even better yet, a video walking me through what I need to do?
If that is not convenient, could you give some more detailed instruction? The part I got lost at was when you said:
Solution: tie that wire via 100 ohm resistor to +12. You can probably omit the resistor, but it is a cleaner solution to limit the current going in to a low current sensor.
Also, since you have been running this setup for 2+ years, have you had any problems or unintended consequences from it?
I would greatly appreciate your help!!
Thanks,
Brian
